Processes for the oligomerization of light olefins to produce C8 olefin oligomers are known. Oligomerization processes have been long employed to produce high quality motor fuel from C4 olefins. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. Indirect alkylation is a noteworthy C4 olefin dimerization process.
In one form of the indirect alkylation process, an ionic exchange resin catalyst oligomerizes light olefins to produce oligomers such as C8 olefins. In such processes, the oligomerization zone can be preceded by a dehydrogenation zone to convert paraffinic feed into olefinic feed or a dehydration zone to convert TBA to isobutylene and/or succeeded by a hydrogenation zone to convert heavy oligomeric olefins into heavy alkanes that can be blended with gasoline stock.
U.S. Pat. No. 4,313,016 discloses a heat exchanged oligomerization reactor that contains a cationic exchange resin catalyst. C4 olefins contacted with the resin catalyst oligomerize to C4 oligomers. Water or methanol may be present in small amounts, insufficient to form an entrained second phase, to serve as a catalyst modifier.
Modern oligomerization processes often include an oxygenate such as tert-butyl alcohol (TBA) and/or sec-butyl alcohol (SBA) in the feed for modifying the catalyst to maintain desired product selectivity. The modifier does not participate in the reaction. References disclosing resin catalyzed oligomerization in the presence of an oxygenate modifier include U.S. Pat. No. 5,877,372 and EP 994 088 A1. TBA and SBA have become the resin catalyst modifier of preference.
In oligomerization processes, it is typically necessary to separate unreacted light olefins from the product heavy oligomers in the effluent from the oligomerization zone. Separation is conventionally performed in a distillation column typically following the oligomerization zone. The lighter components comprising primarily unreacted C4− olefins and compounds that were present in the feed stream exit from the overhead of the distillation column. The heavier components comprising C5+ olefins and primarily oligomers and compounds exit out the bottoms of the distillation column. In U.S. Pat. No. 4,423,264; U.S. Pat. No. 6,011,191 and WO 01/27053, the overhead stream is routed to a second oligomerization reactor followed by a second separation. In U.S. Pat. No. 5,998,685, the overhead stream is recycled to the oligomerization reactor.
An object of the present invention is to utilize at least two reactors for oligomerizing light olefins while sharing one product separation column.
An additional object of the present invention is to take a side draw from the product separation column as feed to the second oligomerization reactor.